The present invention relates to an iontophoresis medication delivery system for controlling infusion of ionic drugs in medical iontophoresis. In particular, the present invention relates to an iotophoresis medication delivery system having an exponentially ramped output current.
Iontophoresis medication delivery systems transfer ionized molecules of a drug solution through a patient's skin. Drug migration is achieved by placing two electrodes on the patient's skin which are connected to an electric direct-current (DC) power supply. One of the electrodes is a source or "active" electrode filled with a drug solution. The other electrode is a return or "inactive" electrode filled with an electrolyte solution. The current delivered to the electrodes generates an electric field between the two electrodes which causes the ionized drug molecules to migrate from the active electrode into the tissues and bloodstream of the patient without the necessity of a hypodermic injection and its adverse effects.
However, patients may experience a shocking or stunning sensation during rapid changes in current level when the delivered current is turned on or turned off. This sensation can be minimized by ramping the current up from zero when the device is turned on and ramping the current down to zero when the device is turned off.
A linear ramp increases and decreases the delivered current slowly over time. An incremental ramp increases and decreases the delivered current in fixed increments over time. While the linear ramp and the incremental ramp reduce some of the discomfort experienced by the patient, the ramps are relatively inefficient since they significantly add to the total treatment time for delivery of a selected drug dose. For example, the linear ramp may take up to one minute to ramp the output current up from zero milliamperes to four milliamperes. If the rate of change of current is increased significantly, the ramp may not have an appreciable effect on patient comfort.
Lattin et al. U.S. Pat. No. 4,406,658 and 4,457,748 disclose an iontophoretic device having a current control circuit which is applied to a pair of electrodes. The current control circuit includes an output transistor having a base connected to a capacitor. A pair of constant current diodes cause the charge on the capacitor and on the base of the output transistor to charge linearly. As the base of the transistor charges, the output current ramps up gradually from zero to a full current value over a period of about one second.
Jacobsen et al. U.S. Pat. No. 4,141,359 discloses an epidermal iontophoresis device which includes a damping circuit to preclude sudden changes in current experienced when the device is turned on or turned off. The damping circuit limits incremental increases of current to minimize discomfort. Newman U.S. Pat. No. 4,931,046 discloses an iontophoresis drug delivery system in which the amplitude level of current pulses is increased in discrete steps (0.1 milliamperes) from a relatively low level to a selected operating level.